1. Technical Field of the Invention
The present invention relates to a semiconductor laser device and, more particularly, to a semiconductor laser device to be used as an optical disk light source of optical communications equipment or optical information equipment for DVDs (Digital Versatile Discs) and CDs (Compact Discs), etc.
2. Description of the Related Art
In general, in order to efficiently radiate heat generated during operation, a semiconductor laser is frequently fusion-bonded and fixed to a high-heat conductivity block. On the other hand, in order to realize a further downsizing of the semiconductor laser device, a semiconductor laser device is demanded which allows a plurality of light sources to be integrated into one package and, furthermore, allows a plurality of light sources to be mounted on one block so as to emit a plurality of laser beams. For example, in recent optical disk systems where light sources are used for reading and writing to CDs and DVDs, an infrared semiconductor laser with both a 780 nm-band wavelength region for CDs with a large light spot diameter and a red semiconductor laser with a 650 nm-band wavelength region for DVDs and having a small light spot diameter are integrated into one package. From the viewpoint of this optical pickup configuration, the light emitting point distance between these semiconductor lasers needs to be approximately 100 μm or less.
A technique for downsizing of semiconductor laser devices, is taught in Japanese Unexamined Patent Publication No. Hei-10-289468. This publication discloses a technique wherein two semiconductor lasers with different wavelengths are arranged side by side and unitized. In addition, Japanese Unexamined Patent Publication No. 2000-11417 discloses a technique wherein a plurality of semiconductor lasers with different wavelengths are build into one chip.
In semiconductor laser devices according to such prior art, two electrodes, which are formed on an insulating block so as to be electrically insulated from each other, and a first semiconductor laser and a second semiconductor laser are arranged on said electrodes via solder or the like, respectively. By employing such a configuration, the first semiconductor laser and second semiconductor laser can each be independently driven on an identical block. Accordingly, it becomes possible to realize a structure wherein the light emitting point distance between these semiconductor lasers is limited only by the distance between two electrically insulated and independent electrodes and the positions of light emitting points on the semiconductor laser end face.
However, in a case where an insulating block or the like is used, material which can be used for this block or the like is restricted by quality and its physical properties. In addition, many materials which may be used for an insulating block or the like have low workability and are difficult to process. Furthermore, in a case where a photodiode for monitoring laser output is provided on an identical block or the like, this photodiode must be additionally mounted, and therefore, the number of components to be mounted on the block is increased, causing an increase in the number of manufacturing steps and manufacturing costs.
For example, as shown in FIG. 1, in a case where a semiconductor laser device is constructed without limitation in material of the insulating block, a dielectric layer 5 is formed on a block 101, and on this dielectric layer 5, a first electrode 61 and a second electrode 62 are formed in a mutually isolated manner, and then, a first semiconductor laser element 31 and a second semiconductor laser element 32 are arranged by joining on the first electrode 61 and second electrode 62 via solder 65, respectively. At this time, if the first electrode 61 and second electrode 62 are out of contact, the respective semiconductor laser element 31 and 32 will be electrically insulated and, therefore, can be independently driven. However, in the case of the semiconductor laser device of FIG. 1, the dielectric layer 5 is interposed between the block 101 and semiconductor laser elements 31 and 32. The dielectric layer 5 generally has low heat conductivity compared to other materials such as a semiconductor and a metal. Therefore, when a semiconductor laser whose heat generation during driving is great or whose heat radiation from its exposed surface is small and a semiconductor laser sensitive to a change in temperature are mounted on the dielectric layer, the temperature characteristics of the semiconductor lasers on this dielectric layer are considerably deteriorated.
On the other hand, Japanese Unexamined Patent Publication No. Hei-5-82904 discloses a technique which provides, although this is a semiconductor laser device provided with a single semiconductor laser, a semiconductor block which functions as a heat radiating member and also as a semiconductor laser light output monitoring photodiode by utilizing an n-type semiconductor substrate for a semiconductor laser mounting block. In this prior art, by forming a p-type semiconductor on a part of the n-type semiconductor block surface, a function such as a photodiode can be provided by this block itself. In addition, on a second portion of the n-type semiconductor block surface a p-type semiconductor is formed, an n-type semiconductor layer is further mounted on this p-type semiconductor surface, and on this n-type semiconductor surface, a semiconductor laser is arranged via an electrode and solder.
Based on this prior art, a semiconductor block functions as a heat radiating member and as a light output monitoring photodiode, and thus a semiconductor laser device mounted with a plurality of semiconductor lasers can also be considered. For example, as shown in FIG. 2, a p-type semiconductor layer 2 is formed on a part of the surface of an n-type semiconductor block 1, and on the surface of this p-type semiconductor layer 2, two n-type semiconductor layers 3 are formed in a mutually separated manner. Then, on one n-type semiconductor layer 3, a semiconductor laser element 31 is arranged via a first electrode 61 and solder 65. In addition, on the other n-type semiconductor layer 3, a second semiconductor laser element 32 is arranged via a second electrode 62 and solder 65. On the upper surface of the respective semiconductor laser elements 31 and 32, a semiconductor laser electrode 24 is formed.
In a semiconductor laser device with the aforementioned structure, the first semiconductor laser element 31 and second semiconductor laser element 32 are electrically insulated by an npn structure composed of one n-type semiconductor layer 3, the p-type semiconductor layer 2, and the other n-type semiconductor layer 3. Accordingly, the respective semiconductor laser elements 31 and 32 can be independently driven. In this semiconductor laser device as shown in FIG. 2, which is different from the semiconductor laser device shown in FIG. 1, because the semiconductor laser elements 31 and 32 are mounted on a semiconductor block higher in heat conductivity than the dielectric layer. Thus, no considerable deterioration in temperature characteristics of the respective semiconductor laser elements 31 and 32 occurs.
However, in the semiconductor laser device shown in FIG. 2, in order to maintain electrical insulating characteristics of the respective semiconductor laser elements 31 and 32, an npn structure must be formed. Therefore, for the light emitting point distance between these semiconductor laser elements 31 and 32, in addition to the distance between the first electrode 61 and second electrode 62 and the positions of light emitting points on the semiconductor laser end face, the distance between the n-type semiconductor layers 3 is determined by the size of the npn structure required for preventing conduction of the respective semiconductor laser elements 31 and 32. Accordingly, the light emitting point distance between the semiconductor laser elements 31 and 32 is unnecessarily expanded, and therein exists a problem.
On the other hand, a domestic republication WO00/04614 of a PCT international publication for patent application as the purpose of providing a semiconductor laser device which can realize an optical pickup capable of reading and writing information with respect to different optical disks by an assembling technique equivalent to that of a conventional optical pickup without increasing the number of components on a heat radiating block. This publication discloses a semiconductor laser device wherein first and second semiconductor laser elements are mounted on a heat radiating block so that, where the distance between an emerging light axis of the first semiconductor laser element and a condenser lens center axis is provided as d1, i.e. the distance between an emerging light axis of the second semiconductor laser element and a condenser lens center axis is provided as d2, and the distance between the emerging light axes of the first and second semiconductor laser elements is provided as L, wherein 0≦L≦d1+d2<160 μm.
Moreover, FIG. 3 shows a semiconductor laser device described in this publication, a sub-mount 132 is mounted on a heat radiating block 131 and semiconductor lasers 133 and 134 are mounted on the heat radiating block 131 and on the sub-mount 132, respectively, so that emerging light axes 135 and 136 of the respective semiconductor lasers become identical in the level of height. Furthermore, a distance therebetween is thus made into a minimum value so as to restrain the distance between the emerging light axes to be a desirable 160 μm or less.
Moreover, in this prior art, for the reason that a light output monitoring photodiode can be formed on the sub-mount, a silicon semiconductor is particularly preferable.
However, in this prior art, no concrete means for enhancing heat radiation characteristics of semiconductor laser elements to be mounted or no concrete means for improving temperature characteristics of semiconductor laser elements is disclosed.